Hi . you will have a filling loop on the system somewhere
It will look like this. With a tap or taps each end . open these until you're pressure gauge reaches 1.2 bar , bleed all the rads then recheck pressure if its dropped then fill to 1.2 again ..
The filling vloop is normally under the boiler but I have known them to be fitted in lofts/ cupboards , next to the hit water cylinder etc
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